1. Field of the Invention
The present invention relates to an ultrasound observation apparatus that generates an ultrasound image using capacitive micromachined ultrasound transducer(s).
2. Description of the Related Art
In recent years, ultrasound observation apparatuses or ultrasound diagnostic apparatuses capable of displaying an ultrasound image of, e.g., a diseased part in a body cavity using an ultrasound probe including ultrasound transducer(s) have widely been used.
Also, capacitive micromachined ultrasound transducers (referred to as C-MUTs) that can easily comply with RoHS, which is a directive issued by the European Union (EU) for restriction of the use of certain hazardous substances in electrical and electronic equipment, and has a broadband characteristic are drawing attention.
As a first related art example, for example, International Publication No. 2006/0411114 discloses an ultrasound observation apparatus in which a DC bias voltage for providing a reference sensitivity is applied during transmission and the DC bias voltage is variably controlled with time during reception. Hereinafter, since DC bias voltage varies depending on the time, the below description will be provided simply using “bias voltage”.
Also, as a second related art example, Japanese Patent Application Laid-Open Publication No. 2006-122344 discloses an ultrasound observation apparatus in which a bias voltage is controlled to gradually increase from a near field to a far field in a receive period.
Furthermore, as a third related art example, U.S. Pat. No. 6,795,374 discloses that a bias voltage is variably controlled according to a function of an image mode according to ultrasound scanning